Many systems have been described for the heterologous expression of nucleic acids encoding e.g. polypeptides such as recombinant proteins in prokaryotic systems. However, most heterologous gene expression systems in prokaryotic host systems have relied exclusively on a limited set of bacterial promoters. The most widely used prokaryotic promoters have included the lactose [lac] (Yanisch-Perron et al., 1985, Gene 33, 103-109), and the tryptophan [trp] (Goeddel et al., 1980, Nature (London) 287, 411-416) promoters, and the hybrid promoters derived from these two [tac and trc] (Brosius, 1984, Gene 27:161-172; Amann and Brosius, 1985, Gene 40, 183-190). Other inducible promoter systems such as the araB promoter inducible by arabinose (WO 86 04356), the rhamnose promoter rhaSB inducible by L-rhamnose (WO 03068956) or the rhamnose promoter rhaBAD inducible by L-rhamnose (WO 2004/050877) have been described as well for the heterologous expression of proteins. WO 2004/050877 describes the use of a rhaBAD promoter for the heterologous expression of nitrilase in E. coli. After induction with L-rhamnose, nitrilase activity in resting-cell assays could be obtained. However, in particular for the expression of complex polypeptides such as antibodies or antibody fragments it is advantageous to export the polypeptide from the cytoplasma to non-cytoplasmic locations (secretion) by using signal sequences, since the overproduction of heterologous proteins in the cytoplasm is often accompanied by the misfolding and segregation into insoluble aggregates (inclusion bodies). However, since the signal sequence can influence secondary and tertiary structure formation in the mature region of secretory polypeptides, the choice of the appropriate signal sequence in combination with an useful promoter is important for high production of functional polypeptides. Thus, there is a need to provide alternative prokaryotic expression systems for the heterologous expression of nucleic acid sequences.